CA1237694A

CA1237694A - Process of producing aluminum substrate for magnetic recording media

Info

Publication number: CA1237694A
Application number: CA000449707A
Authority: CA
Inventors: Koichi Yoshida; Yasuo Oka; Yoshio Hirayama; Takashi Kajiyama
Original assignee: Nippon Light Metal Co Ltd
Current assignee: Nippon Light Metal Co Ltd
Priority date: 1983-03-18
Filing date: 1984-03-15
Publication date: 1988-06-07
Also published as: IT8420101A0; GB2136448A; GB2136448B; DE3408753C2; FR2542901B1; DE3408753A1; IT1206136B; JPH0346890B2; KR840007910A; GB8405910D0; JPS59171023A; NL8400849A; FR2542901A1

Abstract

ABSTRACT OF THE DISCLOSURE

An aluminum substrate suitable for making high-density magnetic recording media is produced by anodising a surface of the aluminum material in an aqueous chromic acid solution at an electrolytic voltage higher than 60 volts. The anodic film thus formed has no black spot defects as also exhibits no cracks when the substrate is heated at a high temperature for forming a magnetic recording layer.
The effect is further improved by adding a small amount of oxalic acid to the electrolyte.

Description

I

PROCESS OF PRODUCING ALUMINUM SUBSTRATE
FOR MAGNETIC RECORDING MEDIA

This invention relates to a process of producing an aluminum substrate for making magnetic recording media, said substrate having an anodically oxidized film or layer on the surface thereof. More particularly, the invention relates to a process of producing an aluminum substrate for making high-density magnetic recording media, said substrate having substantially no black spot defects and having excellent smoothness and heat resistance.
Recently, there has been much demand to increase the recording density of magnetic recording media, such as magnetic disks, etc. For responding to such demands, it is necessary to reduce the thickness of the magnetic medium layer to be formed on a surface of the substrate and the spacing between the magnetic head and the magnetic medium. The substrate of such magnetic recording media is required to have improved surface properties with respect to smoothness and hardness.
As a substrate for making high-density magnetic recording media as above-mentioned, an aluminum substrate having an anodic oxide film on the surface thereof has ',`' I

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been used.
Aluminum substrate having such an anodic oxide film is desirable because the anodic oxide film formed on the surface of aluminum is hard and excellent in wear resist-5 ante, has good polish ability, whereby a smooth surface off accuracy can be easily obtained, and a thin magnetic layer can be easily formed on its surface.
Hitherto, for easily forming an anodlc oxide film having suitable hardness for making magnetic recording media on an aluminum substrate, commonly an electrolytic treatment using a sulfuric acid solution has been per-formed as disclosed in the British Patent No. 1,493,160 of Nov. 23, 1977. However, the aluminum substrate having formed thereon an anodic oxide film using a sulfuric acid solution (hereinafter, is referred to as a sulfuric acid anodic oxide film) has the faults as described hereinafter and these faults are obstacles in increasing the recording density of a magnetic recording media.
One of these faults is a so-called black spot defect. Impurities such as iron, silicon etc., exist-in in aluminum or an aluminum alloy crystallize as inter metallic compounds which exist at the surface of aluminum or the aluminum alloy as microscopic spots.
Such microscopic spots prevent the proper formation of an anodic film during electrolytic treatment. The spots are, at the beginning, very fine ones of sub-micron order but become larger with the growth of the anodic oxide , i., ~3~7~

film and show pit-like fine defects of 5 to 10 m in diameter on the sulfuric acid anodic oxide film having a thickness of 5 em or more. If a substrate has many of these defects, the signal error in the magnetic recording medium is larger and hence the existence of the defect is undesirable.
Another one of the faults is thermal cracking. In the case of the aluminum substrate for high-density magnet tic recording medium, it is required to apply foe lo to the surface of the aluminum substrate by sputtering or vapor deposition, followed by heating to 300-400C for forming foe but when the aluminum substrate is heated to such a high temperature, the sulfuric acid anodic film formed on the substrate is cracked, whereby inferior products are liable to occur. accordingly, the thickness of the anodic oxide film must he reduced Jo a thickness as low as l to 3 my which results in reducing the head crush resistance of the aluminum substrate and hence the occurrence of cracking is undesirable.
As a result of investigation to improve an aluminum substrate having anodic oxide film, the inventors have discovered that when an aqueous chronic acid solution is used for the formation of the anodic oxide film on alum-nut and electrolysis is performed at a constant voltage higher than the voltage employed for conventional chronic acid electrolysis, an aluminum substrate having no black spot defects and no thermal cracking can be produced.

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also, it has been discovered that when in the case of forming the anodic oxide film over 10 em in thickness on an aluminum using the foregoing chronic acid process, a hardness of thus formed film becomes about or over 300 Ho in Tickers hardness, which is requited for head crush resistance and scratch resistance in this kind of aluminum substrate.
SUMMARY OF THE INVENTION
A primary object of this invention is to provide a process of producing an aluminum substrate for magnetic recording medium having formed whereon an anodic oxide film which is hard, is excellent in heat resistance and has practically no black spot defects.
Another object of this invention is to provide an aluminum substrate for a high-density magnetic recording medium.
Other objects and effects of this invention will become apparent from the following detailed explanation.
According to the invention there is provided a process ox producing an aluminum substrate for magnetic recording media, which comprises subjecting an aluminum material to an electrolytic treatment by a constant volt tare process at an electrolytic voltage higher than 60 volts using an aqueous solution of 1.5 to 15% by weight chronic acid as the electrolyte, the temperature of the electrolyte being 30 to 50C.

I

DETAILED DESCRIPTION OF THE INVENTION
An aluminum material used for the substrate in this invention is high-purity aluminum such as aluminum having a playwright of higher than 99.9~ or an alloy composed of the foregoing high-purity aluminum as a base metal containing

2 to 6% by weight magnesium When a high-purity aluminum is used as the aluminum material the aluminum may be clad with an aluminum alloy by rolling, extrusion or lamination with a rigid non-magnetic material such as a hard resin in order to impart a strength to the substrate.
Jo r Such an aluminum material is formed into a eye or any other desired form and after smoothening the surface thereof by grinding or polishing, an anodic oxidizing treatment is applied en the surface ox the aluminum material using a chronic acid-containing electrolyte.
In this invention, the concentration of an aqueous chronic acid solution used as the electrolyte is 1.5 to 15~ preferably 2 - 5%, the temperature of the electrolyte is 30 to 50C preferably 33 to 4?C, and the current density is Cot to 0.8 amp~/dm2 preferably 0.25 - 0.65 amp./dm2, that is, these conditions in this invention are almost the same as the conditions in a conventional constant voltage chronic acid anodic oxidizing treatment called "Buzzard prowesses However, the electrolytic voltage in this invention is higher than the electron lyric voltage, 40 volts in the Buzzard process and is higher than 60 volts, preferably 75 to 100 volts.

Al , ~3~7~

If the concentration of o~alic acid in the electrolyte is over 1/5 of the amount of chronic acid, the thermal cracking resistance of the film formed is reduced and the black spot defect is liable to occur.
S Also, if the amount of the aluminum ion existing in the electrolyte increases over a certain limit during the electrolysis of this invention, the current density suddenly decreases thereby reducing the heat resistance and the hardness of the film formed on the aluminum material. Accordingly, it is desirable to keep the concentration of aluminum ions in the electrolyte below 1/20 of that of chronic acid during the electrolysis.
The aluminum substrate thus obtained has such excellent features suitable for high recording density magnetic recording media that there is practically no formation of fine black spot defects in the aluminum substrate, when the thickness of the film it over 10 em, the hardness of the aluminum substrate is as good as or higher than that of an aluminum substrate treated by a conventional sulfuric acid process anodic oxidation. The aluminum substrate in this invention has excellent polish-ability and scratch resistance properties, and when the aluminum plate treated in this invention is maintained at 300 to 400C for forming a magnetic layer on the surface thereof, fine cracks do not develop owing to the excellent heat resistance of the aluminum substrate.
The inventors' experiments performed for completing lo `,

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this invention will be described hereinafter.
Aluminum-magnesium alloy sheets (Allah My) prepared using high purity aluminum of 99.99% were subjected to an electrolytic treatment in an aqueous 5% chronic acid solution maintained at 35C by constant DO voltage process and the black spot formation on the treated aluminum substrates at each electrolytic voltage was checked, the results being shown in Table 1. Also, the same specimens were heated to 350C for 2 hours, and the formation of fine cracks were examined and the results are shown in Table 1 together with the hardness of the film formed on the sheets in connection with the scratch resistance and the head crush resistance.
Also, for the sake of comparison, the same oval-ration tests as above were applied to an aluminum alloy having the same composition as used in the foregoing tests subjected to a conventional sulfuric acid anodic oxidation process (15~ H2SO4 solution, bath temperature ox 20C, constant voltage process at 10 volts) and the evaluation results are also shown in the same table.
In these tests, the anodic film thickness of the specimens prepared by this invention was 12 em and that of the specimens by the conventional process was 6 em.
Black spot formation was evaluated by the following procedure.

(1) The surface of an aluminum alloy specimen is ''I''' 1 UP) 3 13 planished by D/T (diamond turn) and then sub-jetted to an anodic surface treatment so as to form an anodic film.

(2) The surface of thus formed anodic film is ox-served by a microscope (of 200 magnification and two microscopic visual fields are photo-graphed. The original dimension of one visual field is 0.45 mm2 x 0.4 mm2 = 0.18 mm2 so that two visual fields are of tile dimension of 0.18 my X 2 = Owe mm2.
The dimension of the photograph is 9 cm x 8 cm.
The two visual field are the portions selected optionally on the surface of the anode film, but they are spaced from each other with a distance of at least 1 cm.

(3) The photograph of the two visual fields observed to count the number of the black spot formed on the surface of the anodic film. The black spot is evaluated according to the evaluation Stan dart (of dimension and number) as disclosed in the specification.

In the evaluation of black spot formation shown in Table 1, O signifies that no or one black spot smaller than 2.5 em is observed in the microscopic range of field (0.36 mm2~, US and signifies no or one black spot smaller than 3.5 em - pa -I

in the range above mentioned, while X means the emergency of more or larger black spots.
Also, the evaluation of the formation of fine cracks was made by microscopic observation of the surface of the aluminum substrate after the application of the foregoing heat treatment. In the evaluation shown in Table 1, 0, and X signify no cracking, partial cracking and overall cracking respectively.
The evaluation of the hardness was made using a micro Tickers hardness tester (load 15 g).
Table 1 Voltage (volt) Black spot Crack Hardness ivy _ __ .

~20 _ ~90 Also, aluminum alloy specimens each having the same composition as the aluminum alloy used in the foregoing tests were subjected to an anodical oxidation treatment using an aqueous chronic acid solution having the same temperature and concentration as those in the foregoing tests at constant DO voltage process of 75 jolts to form anodic oxidized films hazing different thicknesses, respectively and the formation ~'7~3~1 of black spot was examined, the results being shown in Table 2.

Then, the aluminum alloy specimens were maintained at 300~C, 350~C, or 400~C for 2 hours and the cracks were examined in each case. The results are also shown in Table 2.

The evaluation modes of the black spot and the hardness are the same as in the foregoing tests shown in Table 1.

Table 2 . _ _ _ . _.

Film Crack thickness _ Black Hardness Ho-(em) 300C350~C 400C spy _ _____ ._ _ __ . _ i _ _ .
6 0 0 0 - 0 ~45 _ . _ _ _ _ _ _ _ __ . .

_ , - _ _ _ . _ __ . _ _ . _ _ Jo _ ., , X _- 2~5 I_ _ _ _ rum the results shown in Table l and Table 2, the following will be recognized.
That is, in the aluminum substrates treated by the process of this invention, it subjected to the electrolytic treatment using an aqueous chronic acid solution by the constant DO voltage process at an electrolytic voltage hither than 60 volts, no or t no black spot formation is observed, cracks are not formed by heating for forming a magnetic layer when the " 'I ' I

thickness of the film is less than about 18 em, and the aluminum substrate satisfies the hardness required for a substrate of this Lund when the film thickness is more than 10~/m~ On the other hand, in the alumimlm substrate subjected to the electrolytic treatment using an aqueous sulfuric acid solution by a conventional typical anodic oxidation treatment or the aluminum substrate subjected to the electrolytic treatment using an aqueous chTomic acid solution under con-ventional conditions, i.e., at a voltage about 40 volts it is difficult to satisfy the three criteria for black spots, cracks, and hardness.
Then, an aluminum-magnesium alloy substrate having the same composition as that used in the foregoing tests was subjected to an electrolytic treatment in each of electrolytic I baths of an aqueous Jo chronic acid solution containing a different amount of oxalic acid at a constant voltage of 90 volts while varying the bath temperature, and the current density (amp./dm2) and the tickers hardness Ho (load: 20 go were measured in each case. The results thus obtained are shown in Table 3, In addition, the thickness of the film formed on the aluminum plate was I m in each case , ~?1,3~

Table -Bath temperature ('C) _ Content of 3 1 I 40 oxalic cold (%) CUD hardness CUD. Hardness CUD. redness 0 0.25 280 0.43 339 0.60 278 _ 0.1 0.28 296 0.44 35 0.62 298 _ .
0.5 0.29 300 0.46 356 0.64 32 (*) : I Current density As shown in the results of the above table, it is under-stood that in the case of using an aqueous chronic acid solution alone, the film formed on the aluminum tends to be softened when the bath temperature is increased but the hard-news of the film can be increased even at a high temperature by adding a small amount of oxalic acid to -the electrolyte and in the case of using an aqueous chronic acid solution containing an oxalic acid, a film having almost the same hardness as in the case of forming a film by using a chronic acid solution can be obtained at a higher bath temperature than that in the foregoing electrolytic treatment, whereby the current density can be increased and the treatment time can be shortened.
As described above, by the process of this invention, the worst problems in the aluminum substrate for magnetic recording Ryder, i.e., -the formation of black spouts, the 1~13 I

formation of cracks at hi~h-temperature treatment, and reduction in head crush resistance and scratch resistance can be solved altogether.
Then, the process of this invention will be further described practically by the following examples.
Example 1 Rater subjecting a disk (inside diameter of 75 mm, outside diameter of 200 my and thickness of 2 mm) of an aluminum alloy (Allah My) prepared using aluminum of owe in purity to an appropriate surface polishing the disk was rinsed in a non-etching decreasing agent and then subjected to an anodic oxidation treatment. That is, the foregoing aluminum alloy disk was immersed in an aqueous owe chronic acid solution maintained at 35C and the electrolytic treat-mint was performed by passing direct current using the aluminum alloy disk as the anode at a constant voltage of 80 volts.
my performing the electrolysis for about 60 minutes at a current density of 0.45 amp./dm2, an aluminum substrate having an opaque and smooth anodic film of 12!~m in thickness was obtained.
Through the performance of electrolysis the concentration of aluminum ion was kept under 2 g/l.
Then, the disk substrate was polished with a grinding medium consisting of fine alumina powder em The surface roughness of polished substrate was 0.02 em (Max.) " , ~,3~7~

After polishing the disk substrate, the hardness thereof was measured by means of a micro-Vickers hardness tester (load: 15 g). The hardness was 30S Ho.
When the surface of the disk substrate was examined with a microscope, no black spot defects were observed.
Also, when the substrate was further heated to okay for 2 hours, no cracks were observed on the surface.
Example 2 After subjecting a disk substrate (inside diameter of 75 my outside diameter of 200 mm, and thickness ox 2 mm) of an aluminum alloy (Al-4% My) prepared using aluminum of owe in purity to an appropriate surface polishing, the aluminum disk substrate was immersed in an aqueous 5% chronic acid c3lution of 35~C containing 1% oxalic acid and the electrolytic treatment was performed at a constant DO voltage of 100 jolts using the disk as the anode to form an opaque and smooth anodic film of 12~.~m in thickness. The current density it the eye ctrolytic treatment was 0.45 amp./dm2 and the treatment time was 45 minutes. Through the performance of electrolysis a concentration of aluminum ion was kept under 2 g/l.
The Tickers hardness (load: 15 g) of the obtained disk substrate after polishing was 351 Ho all when the surface of the disk substrate was examined with a microscope, no black spot defects were observed. Also, when the substrate was further heated to ~50lC for 2 hours, no cracks were observed on the surface '

Claims

Claims:

1. A process of producing an aluminum substrate for magnetic recording media, which comprises subjecting an aluminum material to an electrolytic treatment by a constant voltage process at an electrolytic voltage higher than 60 volts using an aqueous solution of 1.5 to 15% by weight chromic acid as the electrolyte, the temperature of the electrolyte being 30° to 50°C.

2. The process of producing an aluminum substrate for magnetic recording media as claimed in claim 1, wherein the electrolytic treatment is performed by a constant voltage process at a voltage of 70 volts to 100 volts.

3. The process of producing an aluminum substrate for magnetic recording media as claimed in claim 1, wherein the aluminum material is high-purity aluminum having purity of higher than 99.9% or an aluminum alloy composed of the foregoing high-purity aluminum as the base metal and 2 to 6% by weight magnesium.

4. The process of producing an aluminum substrate for magnetic recording media as claimed in claim 1, wherein the electrolyte further contains oxalic acid in an amount not over l/5 of the concentration of chromic acid in the electrolyte.

5. The process of producing an aluminum substrate for magnetic recording media as claimed in claim 1, wherein the electrolytic treatment is performed while keeping the concentration of the aluminum ions in the electrolyte below 1/20 of that of chromic acid.

6. The process of producing an aluminum substrate for magnetic recording media as claimed in claim 1, wherein the thickness of anodic oxide film formed by the electrolysis is from 10µm to 18µm.

7. An aluminum substrate for magnetic recording media, which is produced by a process comprising subjecting an aluminum material to at electrolytic treatment by a constant voltage process at an electrolytic voltage higher than 60 volts using an aqueous solution of 1.5 to 15% by weight chromic acid as the electrolyte, the temperature of the electrolyte being 30° to 50°C.